.TH std::placeholders::_1,std::placeholders::_2,...,std::placeholders::_N 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::placeholders::_1,std::placeholders::_2,...,std::placeholders::_N \- std::placeholders::_1,std::placeholders::_2,...,std::placeholders::_N

.SH Synopsis
   Defined in header <functional>
   /*see below*/ _1;

   /*see below*/ _2;
   .
   .

   /*see below*/ _N;

   The std::placeholders namespace contains the placeholder objects [_1, ..., _N] where
   N is an implementation defined maximum number.

   When used as an argument in a std::bind expression, the placeholder objects are
   stored in the generated function object, and when that function object is invoked
   with unbound arguments, each placeholder _N is replaced by the corresponding Nth
   unbound argument.

   Each placeholder is declared as if by extern /*unspecified*/ _1;.      \fI(until C++17)\fP
   Implementations are encouraged to declare the placeholders as if by
   inline constexpr /*unspecified*/ _1;, although declaring them by       \fI(since C++17)\fP
   extern /*unspecified*/ _1; is still allowed by the standard.

   The types of the placeholder objects are DefaultConstructible and CopyConstructible,
   their default copy/move constructors do not throw exceptions, and for any
   placeholder _N, the type std::is_placeholder<decltype(_N)> is defined, where
   std::is_placeholder<decltype(_N)> is derived from std::integral_constant<int, N>.

.SH Example

   The following code shows the creation of function objects with placeholder
   arguments.


// Run this code

 #include <functional>
 #include <iostream>
 #include <string>

 void goodbye(const std::string& s)
 {
     std::cout << "Goodbye " << s << '\\n';
 }

 class Object
 {
 public:
     void hello(const std::string& s)
     {
         std::cout << "Hello " << s << '\\n';
     }
 };

 int main()
 {
     using namespace std::placeholders;

     using ExampleFunction = std::function<void(const std::string&)>;
     Object instance;
     std::string str("World");

     ExampleFunction f = std::bind(&Object::hello, &instance, _1);
     f(str); // equivalent to instance.hello(str)

     f = std::bind(&goodbye, std::placeholders::_1);
     f(str); // equivalent to goodbye(str)

     auto lambda = [](std::string pre, char o, int rep, std::string post)
     {
         std::cout << pre;
         while (rep-- > 0)
             std::cout << o;
         std::cout << post << '\\n';
     };

     // binding the lambda:
     std::function<void(std::string, char, int, std::string)> g =
         std::bind(&decltype(lambda)::operator(), &lambda, _1, _2, _3, _4);
     g("G", 'o', 'o'-'g', "gol");
 }

.SH Output:

 Hello World
 Goodbye World
 Goooooooogol

.SH See also

   bind           binds one or more arguments to a function object
   \fI(C++11)\fP        \fI(function template)\fP
   is_placeholder indicates that an object is a standard placeholder or can be used as
   \fI(C++11)\fP        one
                  \fI(class template)\fP
   ignore         placeholder to skip an element when unpacking a tuple using tie
   \fI(C++11)\fP        (constant)
